Joel Greenberg once donated $1 million to a struggling after-school music program in West Philadelphia, having read about it in the newspaper.

The Main Line financial trader didn't bother to return a reporter's call asking why he did so.

Greenberg is like that, those who know him say - passionate in his desire to help others, particularly children, and uninterested in self-promotion.

"He does not want to be front and center," said Ina B. Lipman, executive director of the Children's Scholarship Fund, another cause that has benefited from Greenberg's largesse. "He just wants to do good work."

Regardless of his philanthropy, Greenberg's penchant for privacy has proved a distraction for a man who, along with his two business partners, can be expected to play a pivotal role in the Philadelphia mayoral race.

Greenberg, Arthur Dantchik, and Jeffrey Yass, all executives with the Susquehanna International Group in Bala Cynwyd, are about to spend a small fortune to try to elect Democratic State Sen. Anthony H. Williams mayor. After years of avoiding the news media, they now want people to know why.

"We are just very private people trying to do what is right," Greenberg said. "Our goal is purely altruistic: to help the children."

To that end, they are preparing to invest perhaps millions of dollars in an independent campaign to support Williams.

Though Greenberg said he was unsure how much they would spend, he would not discount the possibility that it could approach the $5.38 million they spent backing Williams' unsuccessful gubernatorial campaign in 2010.

The partners will play so dramatic a role on behalf of Williams because of his support of the issue they hold most dear - school choice, particularly for low-income families in Philadelphia.

"Parents should have a choice to send their kids to schools that work," Greenberg said. "They should not be forced to send children to failing, oftentimes violent schools."

That view often has been translated to mean simply more charter schools, and it has earned Greenberg and his partners the enmity of public school advocates and teachers' unions - unfairly so, says Greenberg.

"We are for any school that works," he said, "whether it is a good public school, a good public charter school, a good private school, a good parochial school, a good Jewish day school."

The financial trader met with Inquirer reporters recently as part of a wider outreach to the news media. He was giving the interviews on behalf of all three partners, he said, because questions about their motivations had arisen in the campaign.

Mayoral candidate Jim Kenney, for instance, has labeled the trio "anonymous billionaires more concerned with making a profit than a quality school."

That characterization rankled Greenberg, who denied that he or his partners had any financial stake in this fight.

"They want to create this myth that somehow we are going to create this giant chain of charter schools," he said. "It is not right. . . . We have nothing to do with running charter schools. We don't want to run charter schools."

He described his interest as "purely altruistic." Those who have worked with Greenberg and his partners echo his self-assessment.

"He has no hidden agenda," said Lipman. "He is a very straightforward, committed individual and citizen. He does not have to do the things he does on behalf of other people, but it is part of his DNA.".

That, too, was the read of Montgomery County Commissioner Josh Shapiro, who has worked with Greenberg and his partners on a variety of county issues.

"He is a person who has obviously been very successful in life," Shapiro said. "He recognizes his good fortune and is trying to give back."

Greenberg, 57, traced his "altruism" to his upbringing.

He grew up in a liberal home in Queens, N.Y. His parents were involved in the civil rights movement in the 1960s.

"A big part of the civil rights movement was equal schools," Greenberg said. "If you don't have an education, there is no way you can succeed."

Greenberg attended public schools before getting an undergraduate degree at the State University of New York at Binghamton. He went on to earn a law degree and later worked as a federal prosecutor before transitioning to the world of finance.

He, Yass, and Dantchik were among six founders of the Susquehanna International Group in 1987.

"We were fortunate enough to go into business at the right time," he said. "As we became more successful, we started thinking about how we can give back."

Public schools - particularly those in Philadelphia - quickly became an area of interest.

The district's abysmal graduation and literacy rates were particularly troubling, he said. He, as have others, sees those failures as the root causes of two larger societal problems: crime and unemployment.

"We realized at some point that there would have to be some political change because the system was failing," he said. "About five or six years ago, I was introduced to Senator Williams at a school-choice event. We hit it off."

The millions Greenberg and his partners have funneled into political campaigns since have been driven by the view that good schools should be supported and bad schools shuttered, he said. The type of school is irrelevant.

"No one should be funding failure, whether it's bad public district schools, or bad charter schools," Greenberg said. "There's no reason to keep throwing money continually after failing schools."

He acknowledged the criticism he and his partners have faced had taken a toll.

"It is uncomfortable when I read some of the attacks," he said. "There is some crazy stuff out there. People will sometimes say to me, 'Why do you want to take all this heat? Why not just do what everybody else does in the suburbs, just do your thing?' "

He shrugged.

"I don't know. It is just something that was instilled into us."

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